STARGATE GAME MECHANICS
There are certain mechanical elements in the Stargate roleplaying system that set it apart from the traditional d20 gaming system. Some of these elements are so important that they come up continually and affect the way that other parts of the game work. In order to get a better idea of how the game is shaped, examine the mechanics discussed below.
Action dice are special dice that players and DMs can use to do particular things. Player characters are given a certain number and type of action die at the beginning of each game session. The type of die (d4, d6, etc.) that a character can get is determined by their class level. How many dice a character can have is also determined by level. For instance, a sixth level character will have 4d6 action dice, regardless of their class or species. Dungeon Masters also get a number of action dice to use but they get more dice if there are more players at the table, and if the characters are advanced. When action dice land on their highest number (such as rolling a 4 on a d4), they "explode" and must be rolled again. The results are added together. Because of this, one action die can make a big difference.
Action dice are used for all sorts of special functions. For example, critical hits and critical errors do not happen just because a 20 or a 1 is rolled in Stargate. One action die must be spent to activate a critical hit or error. An action die that is spent in this manner is not rolled, but it is taken out of the action dice pool for the session (so the sixth level character mentioned above would take 1d6 out of their pool of 4d6; they are left with 3d6 to use for the rest of the game session). Players must spend their action dice to activate their own critical hits. Players can also spend action dice to activate NPC critical errors. Likewise, Dungeon Masters can use their action dice to activate player character critical errors.
Action dice can be added to routine dice rolls: to hit an enemy, to determine damage, skill checks, and so forth. Say our sixth level character really needs to hit the goa'uld in front of her. She declares that she is going to add 2 of her 4d6 action dice to her attack roll. She could only use one action die but she's not sure that one will be enough. She could use all of her action dice for this one attack roll but she wants to have some for later on in the session. So, she only uses 2. When she rolls the first d6 she gets a 4. When she rolls the second d6, she gets a 6 and has to roll again. This time she gets a 5. For both of her action dice, she gets a total +15 to her roll.
Sometimes an extra 1d4 can make all the difference between success and failure.
Actions in Combat
In Stargate, a character can take one full action or two half actions in one round. A typical attack is only a half action, which means that most characters are able to attack twice in one round (even at first level). A character can also have as many free actions as the DM will allow, as well as a 5 foot bonus step. It should also be noted that thereare no attacks of opportunity in the Stargate system.
There is a plethora of gear in the Stargate game. This gear is arranged, for the most part, into a series of bundles arranged around a task, from the archaeologist bundle to the Mr. Fix It bundle. This makes it easier to assign gear at the start of each mission, because related items are given out together. Individual items can be had outside of bundles if gear picks are spent for them, and the gear in bundles can be switched out if necessary (if a bundle comes with a weapon a character isn't proficient with, say).
Stargate travellers face great peril even without combat. To reflect this, various dangers are detailed and given challenge ratings. Characters are entitled to gain experience from encountering and being challenged by such things. Environmental dangers like lava can hamper and even kill characters, and weather conditions can be just as troublesome. Diseases and poisons are perilous on any planet, even Earth. Traps and sensors are also taken into account, since getting past security can be quite a feat, especially in the Stargate milieu.
There are a host of combat options in Stargate. Some of these options are familiar, since they are taken from the d20 system. Options like fluid initiative might not be as familiar; even though this initiative system can be found in the DMG, it is presented as a variant. It is intended that groups will roll initiative each round in Stargate, and that modifiers will be applied for favorable or negative conditions. It is not presented here as a variant but is instead the order of the day.
There are a lot of non-offensive options available in Stargate. There are various types of crouches that a character can get into, each with its own benefits. It is also possible for any character to threaten or taunt opponents. Tripping, disarming, and grappling maneuvers are still available, as well.
Guns feature heavily into the available attack options, since they are favored by the SGC and are more versatile than less technical weapons. These options are limited, however, by the type of gun involved. Not all models are able to perform an autofire or burst attack, for example. However, all guns enable a character to provide cover fire for teammates, or lay down suppressive fire to pin enemies in place.
Critical Success and Failure With Skills
In the average d20 system, you either succeed or fail at skill checks. A critical success doesn't do much extra, and a critical failure doesn't mean much in and of itself. You either succeed, or you don't.
In the Stargate game, critical successes and failures matter, even with skills. By activating a critical success on a skill roll, a character will be able to use the skill better than normal. A critical error can be activated by the DM to make the character fail in a particularly spectacular way.
The default for a critical success is to roll a 20 on a d20 and spend an action die. The default for a critical failure is to roll a 1 on a d20, and the DM spends an action die. But these values can change due to feats or special abilities. So, for instance, a character's critical success range with a skill could be altered so that they can roll a 19 or a 20 (and then spend an action die). Or they could have an error range of 1-2 with a skill. This makes skills more versatile in the long run.
It is possible to do all sorts of things with the gear in Stargate, above and beyond standard uses.
First of all, there are rules for concealing pieces of gear (which includes weapons) on a character's person or in an area. In standard d20 fantasy games, most characters are able to carry their weapons around openly and they do so. In Stargate, carrying weapons in the open is not always the best option. The nice news is that any character can attempt to hide gear - or to find it.
Gear can also be damaged in the Stargate system. There are mechanics to cover how sturdy objects are and the different levels at which they can be damaged while still functioning (called damage thresholds).
Gear can be repaired by characters, and since the setting of Stargate is often rife with technology this can become a big bonus. Three separate skills can be used to repair the various gear in Stargate: Computers, Electronics, and Mechanics.
It must also be noted that the gear in the game has all sorts of qualities. Some qualities come with an item automatically because of the way it is made (like a taser being battery operated). Other qualities can be added to items for a cost in gear picks. In this way, weapons, armor, and other objects can be custom-outfitted.
The standard d20 rules do not always apply to skills or other game elements in Stargate. Many of the skills have been reworked for the modern setting. Other things, like attack types, have been altered for other reasons. It is important to read descriptions carefully even if you think you know what the rules should be because Stargate is more than the d20 system.
Vitality and Wound Points
Combat in the Stargate game is modeled after combat on the television show in one big way: the characters tend to get worn down but are not directly hurt that often. When they are hit, it tends to be serious business.
When a character is hit by an attack, their vitality points get taken away first. This does not mean that they have sustained bodily injury, however; the book describes it more as getting slowly worn down by the situation. When all of a character's vitality points are gone, they get tired and are less effective. They're not out of the fight yet, however - they still have their wound points to go through. Once they are out of wound points, they are unconscious, and if they are at negative wound points then they are dying (as per the standard d20 rules). When a critical hit is activated, the damage is not applied to vitality points at all, but goes directly to wound points. This makes critical hits more dangerous to a character's health but is in keeping with the way the television show works.